This invention relates to AC electric energy meters which include a programmable time-based electric energy measuring system for performing various metering operations and relating those operations to realtime. More particularly, the invention relates to power outage recovery circuits for use with electric energy meters of this type.
Electric energy meters are extensively used for measuring electric energy usage at an AC power consumer's facility. Several types of meters may be used at the facility, including demand registers, time-of-use meters and tariff registers. Tariff meters or registers establish the amount of electric energy consumption for billing by the electric energy supplier. Most tariff meters provide for measuring the total comsumption of electric energy over a billing perid or for measuring a maximum or peak demand or usage during a particular time period. Many tariff meters are capable of performing AC energy comsumption or useage calculations at specific intervals or on the occurrence of specific time-related events.
In recent years, the electric energy meters have moved away from the traditional electromechanical type of meters to the more sophisticated and flexible electronic demand and tariff registers. These electronic registers measure AC electric energy usage in a manner similar to that of the prior electromechanical types. The electronic registers present the advantage of being able of program specific time-related events for purposes of adjusting billings or adjusting usage calculations. The electronic demand registers may also be pre-programmed to respond at specific time intervals in order to calculate and/or communicate various time-of-use information. Most frequently, these time-related events are stored in a memory in the meter electronics as clock calendar times--that is, by second, minute, hour, day and date. One disadvantage of the electronic register is that the register is susceiptible to problems in the event of an AC power outage or power failure. Since the electronic demand registers include electronic circuitry and electronic memories, some DC power is required by the electronic components in order to function or to maintain data stored within electronic memory.
Various devices of the prior art have suggested means for accounting for power outages at the electric energy meter. Particularly, these prior devices provide some means for maintaining the functions of the electrical components at a reduced current level, such as through a secondary DC power supply. Many of these devices utlize non-volatile memory for the storage of critical times or time-based information. For instance, the patent to Johnston, et al., U.S. Pat. No. 4,197,582, shows an electric energy meter which includes a non-volatile data memory for processing time-related data and which further includes an auxiliary DC supply energized by a battery which is used to power a low power secondary timer to maintain the current time and calendar registrations during the power outage. The patent to Stokes, et al., U.S. Pat. No. 4,701,858, illustrates another example of a electronic time-of-use utility meter having a non-volatile realtime clock calendar module for maintaining realtime during a power outage.
Other devices using similar power outage and power backup schemes are shown in the following patents: Sciulli, U.S. Pat. No. 4,495,596; Jerrim, U.S. Pat. No. 4,335,447; Dimassimo, et al., U.S. Pat. No. 4,465,970; and Wolf, U.S. Pat. No. 4,291,375.
The approaches taken in many of the prior art devices have several shortcomings. For instance, the devices that utilize a lower power timer to periodically perform housekeeping functions in the electronic demand register do not include a "catchup" mode in which the register, in essence, replays the time lost during the power outage in order to determine whether a time-related event has occurred. Other proposed solutions, such as a separate clock calendar computer chip are expensive and require a larger meter package.
In view of these and other limitations of proposed prior art solutions, there is a need for an electric energy meter having power fail timekeeping and recovery functions that is inexpensive and easily packaged, preferably as software rather than hardware. In addition, there is also a need for an electric energy meter with these functions that is capable of accounting for all time-related events that may have occurred during a power outage.